The burden of the demands for the improved performance of computers and other electronic devices falls on the lithographic processes used to fabricate integrated circuit chips. Lithography involves irradiating a mask and focusing the pattern of this mask through an optical microlithography system onto a wafer coated with a photoresist. The pattern on the mask is thereby transferred onto the wafer. Decreasing the line-widths of the features on a given wafer brings about advances in performance. The enhanced resolution required to achieve finer line-widths is accomplished by decreasing the wavelength of the illumination source. As a result, the energies used in lithographic patterning are moving deeper into the UV region. Consequently, optical components capable of reliable performance at these short optical microlithography wavelengths are required.
Few materials are known that have a high transmittance at wavelengths below 200 nm, for example, at 193 nm and 157 nm, and also do not deteriorate under intense laser exposure. Fluoride crystals such as those of magnesium fluoride, calcium fluoride and barium fluoride are potential materials that have a high transmittance at wavelengths <200 nm. Projection optical photolithography systems and <200 nm excimer laser systems that utilize vacuum ultraviolet wavelengths of light at and below 200 nm provide desirable benefits in terms of achieving smaller feature dimensions. Consequently, excimer lasers and microlithography systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits and their manufacture.
The commercial use and adoption of 193 nm and below vacuum ultraviolet wavelengths (“VUV”), for example the 157 nm wavelength region, has been hindered by the transmission nature of such deep ultraviolet wavelengths through optical materials. The slow progression in the use of VUV light below 200 nm (e.g., 157 nm region light) by the semiconductor industry has been also due to the lack of economically manufacturable, high quality blanks of optically transmissive materials suitable making below 200 nm microlithography optical elements. Consequently in order to utilize deep ultraviolet photolithography to manufacture integrated circuits using, for example, fluorine excimer lasers, there is a need for below 200 nm wavelength transmitting optical fluoride crystals such as those made from the fluorides of magnesium, calcium and barium. Such crystals must be of high quality, possess few defects and have beneficial, highly qualified optical including low contaminant levels, and be of low birefringence.
The process of growing large metal fluoride single crystals that are relatively free of defects typically lasts several weeks; particularly for high optical quality fluoride single crystals and below 200 nm optical lithography single crystal blanks. The cost of the equipment to produce high quality crystals, as well as the staging of the crystal-growth process, is high and there is no guarantee of having a successful result at the end of the growth process. Therefore, there has long been a concerted effort to increase the yield of high quality metal fluoride single crystals.
Optical crystals can be grown by any of several methods known in the art; for example the Stockbarger, Bridgman-Stockbarger and Czochralski methods, and the methods described in U.S. Pat. Nos. 6,630,117, 6,395,657, and 6,333,922 B1. In the Bridgman-Stockbarger method, the optical crystals are grown in a vertical furnace by moving molten crystal material through a temperature gradient zone in the furnace. There are typically two temperature zones in the furnace; an upper or growth zone and a lower or annealing/cooling zone.
Crystals grown using Bridgman-Stockbarger method are exposed to sharp localized cooling as they are translated through a temperature gradient zone from the upper zone into the lower zone. The sharp localized cooling that occurs during this translation induces permanent thermal strain (stress) in the crystals that consequently gives rise to unacceptably elevated birefringence values in the crystals. In order to reduce the permanent thermal strain in the crystal, the crystal is annealed in the lower zone of the growth furnace. This annealing in the lower zone is generally known as “primary” annealing and is conducted for a selected time over a selected temperature range from less than that of the melting point of the metal fluoride down to ambient temperature. (A number of different time/temperature profiles for primary annealing are known in the art.) After the annealing is complete and the crystal has been cooled to near ambient temperature, it is removed from the growth furnace and the birefringence of the crystal is measured. If the crystal has an unacceptably high birefringence value, the crystal is further annealed in a separate furnace from the growth furnace. This second annealing process is generally referred to as “post-annealing” or “secondary annealing”. (A number of secondary annealing profiles are also known in the art, one example being U.S. Pat. No. 6,332,922.) Metal fluoride crystals grown using the Bridgman-Stockbarger or other process are usually grown as cylindrical disks or ingots that are subsequently processed and shaped into optical elements.
The crystal surfaces (the as-grown surfaces of the crystal) are formed during the crystal growth and primary annealing process. These as-grown surfaces consist of a layer of graphite particles, gas bubbles, smaller metal fluoride crystals and other optically opaque compounds on their surface. This layer of material (metal fluoride, graphite, bubbles, and distinct small metal fluoride crystals) has significantly different thermal properties such as emissivity, absorption coefficient, specific heat, thermal conductivity and density as compared to a pure metal single crystal surface that doers not contain such substances. Based on actual observations, the compositions and structure of the as-grown surfaces are not consistent as between the top surface, the bottom surface and the side surface of a cylindrical metal fluoride disc. They may also vary from disk-to-disk and from run-to-run. These variations of the as-grown crystal surfaces are one of the major causes of crystal quality variations. Consequently, there is need to have a better method of growing metal fluoride single crystals; and particularly for a method that will reduce birefringence and crystal-to-crystal variations in quality.